ST. LOUIS, Mo. — The nation’s worst drought in decades has claimed its latest victim: Part of the busiest lock on the Mississippi River.

Because of low river levels – which stand in stark contrast to last year’s record flooding along the Mississippi – part of a protective barrier at Lock 27 near Granite City, Ill., just upriver from St. Louis, was significantly damaged.

The rock-filled barriers sit at the front of the lock and are used to help vessels get aligned as they enter the structure. Most of them usually sit under water, but with this season’s extreme drought, an extra 15-20 feet of the barrier has been exposed, said Mike Peterson, a spokesman for the U.S. Army Corps of Engineers.

On Sept. 16, one of the barriers split open, Peterson said. The rock inside spread out into the river and blocked passage to the lock. At one point while the lock remained closed, a total of 455 barges on 63 vessels were stalled while the Corps worked for nearly five days to repair the barrier and clear the opening.

The backlog included grain, coal and other items that would have filled about 26,400 tractor-trailers or more than 6,000 railcars. The Corps and U.S. Coast Guard reopened the lock last Thursday morning, but the last of the stalled barges wasn’t expected to make it through the lock until this past Sunday or Monday at the earliest, said Coast Guard Lt. Colin Fogarty.

“While it may seem slow, it’s actually quite a good pace given the dynamic nature of the river and the pure size of the vessels,” he said.

The delay has been costly to the shipping industry, Peterson said. Roughly half of the nation’s grain exports – an estimated 75 million tons a year – move along the Mississippi and through Lock 27.

Each day the barges were idled because of the barrier breach cost shipping companies an estimated $2 million-$3 million, Peterson said. “When you put a chokehold on Lock 27, it really affects the main artery through the American transit system,” Fogarty added.

The drought’s impact since July has slowed barge traffic along the Mississippi and Ohio rivers, which have seen levels down as much as 25 feet in some spots.

Shipping companies have been forced to lighten some of their loads to avoid getting stuck in some spots, and the Corps has occasionally closed parts of the Mississippi for days at a time this season.